How to find the ideal wheel and tire size for your track car | Articles

Miata Tire Test Times

But every tire is different, and the A052 is typically not happy being pinched. Of course, with the available A052 tire sizes for our CRX, pinching was not going to be a factor. In fact, the 205s were fairly stretched on 9-inch-wide wheels, with the 215s and 225s being a normal straight-up fit.

CRX Tire and Wheel Specs

Now to see which tires our CRX preferred. One logistical shortcut we took was to change only the front tires for each round of testing–the rears were kept constant at 205/50R15. 

With our nose-heavy, front-drive application, the fronts do most of the work anyway, so this shortcut has served us well in the past. It did make for some rather odd-looking combinations, though. 

From left to right: 205/40R17, 215/40R17, 225/50R15, 205/50R15

As we always do, we heat-cycled our tires prior to test day. In this instance, the entire operation was done at the track. We built heat into them by doing some skidpad circles in an empty paddock area before running six laps on each set of tires. Tires were then removed and allowed to cool for 24 hours, better curing the rubber for optimal performance.

Not all tests go as planned. We typically do our work on weekday mornings when the track is mostly empty. This minimizes track evolution beyond an initial dust-off, and also allows for uninterrupted lapping for evaluating heat-sensitive tires. But this weekday was different, as a slew of cars began to fill the paddock not long after the track opened. Still, we were hopeful we could find gaps in the flow to make it work.

• size: 205/50R15 
• best lap: 1:24.3

After 20 minutes of lapping on some old scrubs to clean the line and get our shifting and braking points dialed in, we mounted up the 205/50R15s to set our baselines. With the A052 at full tread, the first lap is typically the quickest. Heat soak sets in soon after, dropping the performance potential. 

• size: 205/40R17
• best lap: 1:23.9

Our next size was a little wider but also had much shorter sidewalls. Compared to the 15s, these were definitely quicker to respond to driver inputs, counteracting some of the A052’s typical vagueness. The car also had less torque steer and felt steadier over bumps, as the 17-inch wheels had 8mm less offset. This reduces the scrub radius and is possible because the front suspension upright is curved, allowing for more clearance the farther from the center you go. In short, the car felt more natural and the times reflected that. 

• size: 215/40R17
• best lap: 1:23.4

Add a little more tread width to all that 17-inch goodness and you get the 215/40R17. Grip was up, and lap times dropped some more. 

• size: 225/50R15
• best lap: 1:23.2

Our final comparison size was 225/50R15. By now, traffic was becoming a major problem, with gaps becoming few and far between. This session got cut short as a large group of cars entered the track just after our second hot lap.

Before calling any test final, we always retest the first setup. Why? We want to make sure track evolution and driver improvement haven’t contaminated the data.

So we again mounted up our 205/50R15 baseline tires for that final bracket session. And then came a dose of reality: Traffic wouldn’t die down enough for a clean session. We waited and waited–for 2 hours. And then, as quickly as they had arrived, everyone else left. 

Heading back out, we were greeted with a completely different track surface. Due to all the extra cars on track, grip was clearly up in many key areas, with our lap time plummeting by a full second. This is not even close to what we were looking for in a confirmation bracket session.

Could our earlier data be trusted? Or was the pace increase progression simply the track getting faster as we swapped to ever larger tires? Our intuition said we were fine, but the science called for a do-over.

So we did it all again.

Round 2

With limited time on hand, we could only do three more sessions. Since the 205/40R17 wasn’t as fast as the 215mm and 225mm tires, we skipped that one. Our new, faster 205/50R15 times would serve as the baseline for this round, and the final retest session on that tire duplicated the baseline nicely. Success!

So What Did We Learn?

Photography Credit: Anna Overman

For sure, we confirmed a lesson we’d already known: A wider tire is always quicker if properly supported by a wide enough wheel. Or, as we saw with the Miata test, were the faster laps from the taller diameter of our wider tires?

Given the limited sizing availability, those two variables could not be fully isolated. Either way, we now had two good choices for tire sizes for our One Lap of America journey. How do we decide between them? One more round of testing.

Having now verified fit and function, we headed back to the track the following week with just the two larger candidates. This time, we really focused on the subjective strengths and weaknesses of each combination in addition to pure lap times. Here’s that data:

While the 17s again felt a bit more natural due to the stock-like offset of the wheel fitment, the short sidewalls made the tires more edgy at the limit. Further, the reduced amount of rubber in the 17-inch setups contributed to earlier heat soak. All that energy generated during a hot lap has to go somewhere, and the tire is that place.

As a result, the big 15s were easier to consistently drive fast. And with One Lap time trials being a three-lap affair and every circuit being scored, consistency is key. 

Back to our original question. How do you determine the best wheel-and-tire setup for your car? Ideally with some methodical testing.